Abstract

The Hellefjord Schist, a volcaniclastic psammite-pelite formation in the Caledonides of Arctic
Norway contains discoidal impressions and apparent tube casts that share morphological
and taphonomic similarities to Neoproterozoic stem-holdfast forms. U-Pb zircon
geochronology on the host metasediment indicates it was deposited between 437 ± 2 and
439 ± 3 Ma, but also indicates that an inferred basal conglomerate to this formation must be
part of an older stratigraphic element, as it is cross-cut by a 546 ± 4 Ma pegmatite. These
results confirm that the Hellefjord Schist is separated from underlying older Proterozoic
rocks by a thrust. It has previously been argued that the Cambrian Substrate Revolution
destroyed the ecological niches that the Neoproterozoic frond-holdfasts organisms occupied.
However, the discovery of these fossils in Silurian rocks demonstrates that the environment
and substrate must have been similar enough to Neoproterozoic settings that
frond-holdfast bodyplans were still ecologically viable some hundred million years later.